How to Tell If Something Is Moving or Still

An object is said to be at rest if its position does not change with respect to a chosen reference point, and in motion if its position changes over time. To decide if something is moving or still, you always compare its position to another object or place, called the reference point. If the distance between the object and the reference point stays the same, the object is at rest. If the distance changes, it is in motion. For example, if you are sitting in a parked bus at a bus stop, you are at rest with respect to the seat, but if the bus starts moving and you see trees passing by outside, you are in motion with respect to the trees. In real life, a book lying on a table is at rest with respect to the table, but a person walking past the table is in motion with respect to the book. A common mistake is to forget that motion and rest are always described relative to something else—there is no absolute rest or motion.

Motion can be classified based on the path taken by an object: rectilinear (straight line), circular (along a circle), or oscillatory (to and fro about a fixed point). In rectilinear motion, an object moves in a straight path, such as a train moving on straight tracks. Circular motion happens when an object moves along a circular path, like the hands of a wall clock. Oscillatory motion is when an object swings back and forth around a central point, such as a pendulum in a wall clock. For example, a child sliding down a straight slide shows rectilinear motion, a spinning top shows circular motion, and a child on a swing shows oscillatory motion. Sometimes, objects can show more than one type of motion at the same time, which can be confusing if you only look at one part of the object.

Everyday objects often show one or a combination of different motions. To identify the type, observe the path traced by the object or its parts. A moving bicycle wheel shows both circular motion (the rim rotates around the axle) and rectilinear motion (the wheel moves forward in a straight line). A swinging pendulum displays oscillatory motion as it moves back and forth about its mean position. The blade of a ceiling fan shows circular motion as it rotates around its center. A marching child moving straight ahead shows rectilinear motion. For example, if you look at the tip of a fan blade, it moves in a circle, but the whole fan is fixed at one place. If you watch a child march in a parade, their path is straight, which is rectilinear. Remember, sometimes a single object can have different motions at the same time—like the bicycle wheel example—so always look at the part of the object and its path.

To calculate the distance moved by an object along a straight line, subtract the initial position from the final position. The formula is: Distance moved = Final position – Initial position. Both positions must be measured from the same reference point and in the same unit (such as metres or centimetres). For example, if a toy car starts at 2 metres from a wall and moves to 7 metres from the same wall, the distance moved is 7 m – 2 m = 5 m. In real life, if a student walks from their classroom door (0 m) to the school gate (50 m), the distance moved is 50 m – 0 m = 50 m. A common mistake is to forget to use the same reference point or unit for both measurements, which can lead to wrong answers.